Projection optical system, exposure apparatus and method using the same

ABSTRACT

A projection optical system includes an optical element that includes and locally uses a reflective or refractive area that is substantially axially symmetrical around an optical axis, the optical element being rotatable around the optical axis.

This application is a continuation application of U.S. patentapplication Ser. No. 10/407,721 filed on Apr. 4, 2003, which issued asU.S. Pat. No. 7,006,194 on Feb. 28, 2006, which is herein incorporatedby reference.

BACKGROUND OF THE INVENTION

The present invention relates to an exposure apparatus for exposing anobject, such as a single crystal substrate and a glass plate for aliquid crystal display (“LCD”). The present invention is especiallysuitable for an exposure apparatus that uses ultraviolet (“UV”) andextreme ultraviolet (“EUV”) light as an exposure light source.

Along with the recent demand on smaller and lower profile electronicdevices, a miniaturization of semiconductor devices to be mounted ontothese electronic devices have been increasingly demanded. For example, adesign rule for a mask pattern requires that an image with a size of aline and space (L & S) of less than 0.1 μm be extensively formed, andpredictably, it will further move to a formation of circuit patterns ofless than 80 nm in the future. L & S denotes an image projected to awafer in exposure with equal line and space widths, and serves as anindex of exposure resolution.

A projection exposure apparatus, which is a typical exposure apparatusfor fabricating semiconductor devices, includes a projection opticalsystem that projects and exposes a pattern formed on a mask or a reticle(which are used interchangeably in the present application) onto awafer. Resolution R of a projection exposure apparatus (i.e., a minimumsize which enables a precise transfer of an image) can be given by usinga light-source wavelength λ and the numerical aperture (NA) of theprojection optical system as in the following equation:

$\begin{matrix}{R = {k_{1} \times \frac{\lambda}{NA}}} & (1)\end{matrix}$

As the shorter the wavelength becomes and the higher the NA increases,the better the resolution becomes. The recent trend has required thatthe resolution be a smaller value; however it is difficult to meet thisrequirement using only the increased NA, and the improved resolutionexpects use of a shortened wavelength. Exposure light sources havecurrently been in transition from KrF excimer laser (with a wavelengthof approximately 248 nm) and ArF excimer laser (with a wavelength ofapproximately 193 nm) to F₂ excimer laser (with a wavelength ofapproximately 157 nm). Practical use of the EUV light is being promotedas a light source.

As a shorter wavelength of light limits usable glass materials fortransmitting the light, it is advantageous for the projection opticalsystem to use reflection elements, i.e., mirrors, instead of using manyrefraction elements, i.e., lenses. No applicable glass materials havebeen proposed for the EUV light as exposure light, and a projectionoptical system could not include any lenses. It has thus been proposedto form a reflection type reduction projection optical system only withmirrors. In the reflection type or catoptric optical system, respectivemirrors and a surface shape of each mirror are arranged axiallysymmetrical around one optical axis.

However, an area or slit used for exposure is a limited area that isapart from the optical axis, and one or more mirrors in the catoptricoptical system receive exposure light locally or partially. Such amirror as locally receives the exposure light generates a temperaturedifference in its materials, deforms its surface shape, and deterioratesits optical performance. Accordingly, Japanese Laid-Open PatentApplication No. 11-243052, for example, discloses a catoptric projectionoptical system provided with a cooling unit for mitigating a temperaturedifference on the mirror.

However, the catoptric projection optical system proposed in JapaneseLaid-Open Patent Application No. 11-243052 provides the cooling unit atthe rear side of an unilluminated area and reflection surface of amirror that uses only part for exposure, and thus the temperaturedifference generated in the mirror cannot be mitigated sufficiently.After all, the surface shape changes and deteriorates the opticalperformance, and this system cannot obtain desired resolution.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an exemplified object of the present invention toprovide a projection optical system, and an exposure apparatus andmethod, which may obtain desired resolution, and provide superiorexposure performance.

A projection optical system of one aspect of the present inventionincludes an optical element that includes and locally uses a reflectiveor refractive area that is substantially axially symmetrical around anoptical axis, said optical element being rotatable around the opticalaxis.

The projection optical system may include a cooling unit that cools thearea directly. The area may have a surface shape that changes aberrationwhen said optical element rotates. The aberration may include at leastone of a curvature of field, distortion and coma. The projection opticalsystem may further include another axially asymmetrically shaped opticalelement, said optical element rotating and reducing aberration caused bysaid other optical element. The aberration may include astigmatism. Theoptical element may have a hole at a center thereof, through which lightpasses. The projection optical system may use light having a wavelengthof 20 nm or smaller. The optical element may have an aspheric surface.

An exposure method of another aspect of the present invention includesthe steps of evaluating a deterioration of a projection optical systemfor projecting, onto an object to be exposed, light from a mask whichforms a pattern, selecting, based on a result of said evaluating step, apredetermined area on the optical element for correcting thedeterioration of the projection optical system, and exposing the objectto be exposed using the predetermined area.

The evaluating step may evaluate based on exposure does of thepredetermined area, an exposure result of the object, or aberration thatoccurs in the projection optical system. The selecting step may selectthe area by rotating the optical element or by selecting one of pluraloptical elements. The selected one of plural optical elements may have areflection element having an aspheric surface.

An exposure apparatus of another aspect of the present inventionincludes the above projection optical system.

A device fabricating method of another aspect of the present inventionincludes the steps of exposing an object using the above exposureapparatus, and performing a predetermined process for the exposedobject. Claims for a device fabricating method for performing operationssimilar to that of the above exposure apparatus cover devices asintermediate and final products. Such devices include semiconductorchips like an LSI and VLSI, CCDs, LCDs, magnetic sensors, thin filmmagnetic heads, and the like.

Other objects and further features of the present invention will becomereadily apparent from the following description of the preferredembodiments with reference to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing an exposure apparatus ofone embodiment according to the present invention.

FIG. 2 is a schematic sectional view showing a projection optical systemof one embodiment according to the present invention.

FIG. 3 is a plane view showing an exemplary optical element shown inFIG. 2.

FIG. 4 is a plane view showing the exemplary optical element shown inFIG. 2.

FIG. 5 is a plane view of an optical element that is rotatable around anoptical axis and has a surface shape for correcting a deterioration ofanother optical element due to a change of surface shape of the otheroptical element.

FIG. 6 is a flowchart for explaining an exposure method according to thepresent invention.

FIG. 7 is a flowchart for explaining a method for fabricating devices(semiconductor chips such as ICs, LSIs, and the like, LCDs, CCDs, etc.).

FIG. 8 is a detailed flowchart for Step 4 of wafer process shown in FIG.7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given of a reflection type projection opticalsystem 300 and an exposure apparatus 200 as one aspect of the presentinvention with reference to the accompanying drawings. The presentinvention is not limited to these embodiments and each element isreplaceable within a scope that achieves the objects of the presentinvention. Here, FIG. 1 is a schematic structural view showing theexposure apparatus 1 of one aspect according to the present invention.

The exposure apparatus 1 includes, as shown in FIG. 1, an illuminationapparatus 100, a mask 200, a projection optical system, a plate 400, anda controller 500.

The exposure apparatus 1 uses, as illumination light for exposure, anEUV ray (with a wavelength of, e.g., 13.4 nm) to exposes onto the plate400 a circuit pattern formed on the mask 200, for example, in astep-and-repeat manner and step-and-scan manner. This projectionexposure apparatus is suitable for a lithography process of a submicronor a quarter-micron or less, and thus a description will be given of astep-and-scan type exposure apparatus (also called as a “scanner”) as anexample in this embodiment. Here, the “step-and-repeat manner” is onemode of exposure method that moves a wafer stepwise to an exposure areafor the next shot every shot of cell projection onto the wafer. The“step-and-scan manner,” is another mode of exposure method that exposesa mask pattern onto a wafer by continuously scanning the wafer relativeto the mask, and by moving, after a shot of exposure, the wafer stepwiseto the next exposure area to be shot. Since the EUV light has lowtransmittance for air, at least the optical path through which the EUVlight travels is desirably accommodated in a vacuum atmosphere, althoughnot shown in FIG. 1.

The illumination apparatus 100 uses an EUV ray with a wavelength lessthan 20 nm (with a wavelength of, e.g., 13.4 nm) to illuminate the mask200 that forms a circuit pattern to be transferred, and includes a lightsource part 110 and an illumination optical system 120.

The EUV light source 110 uses, for example, a laser plasma light source.The laser plasma light source irradiates a highly intensified pulselaser beam to a target material put in vacuum, thus generatinghigh-temperature plasma for use as EUV light with a wavelength of about13.4 nm emitted from this. The target material may use a metallic thinfilm, inert gas, and droplets, etc. The pulse laser preferably has highrepetitive frequency, e.g., usually several kHz, for increased averageintensity of the emitted EUV light. Alternatively, the EUV light source110 may use a discharge plasma light source, which emits gas around anelectrode put in vacuum, applies pulse voltage to the electrode fordischarge, and induces high-temperature plasma. This plasma emits theEUV light, for example, with a wavelength of about 13.4 nm to beutilized. Of course, the light source part 110 is not limited to them,but may use any technology known in the art.

The illumination optical system 120 propagates the EUV light,illuminates the mask 200, and includes a condenser optical system, anoptical integrator, an aperture stop, a blade, etc. For example, thecondenser optical system includes one or more mirrors for condensing EUVlight that is radiated approximately isotropically from the light sourcepart 110, and the optical integrator uniformly illuminates the mask 200with a predetermined aperture.

A debris eliminator (not shown) is preferably arranged between the lightsource part 110 and the illumination optical system 120 to eliminatedebris generated concurrently when the EUV light is produced.

The mask 200 is a catoptric mask or a transmission type mask, such as adie-cutting mask, and forms a circuit pattern or image to betransferred. It is supported and driven by a mask stage 250. Thediffracted light emitted from the mask 200 is projected onto the plate400 after reflected by the projection optical system 300. The mask 200and plate 400 are arranged optically conjugate with each other. Sincethe exposure apparatus 1 of this embodiment is a scanner, the mask 200and plate 400 are scanned to transfer a pattern on the reticle 220 ontothe plate 230.

The mask stage 250 supports the mask 200, and is connected to a movingmechanism (not shown). The mask stage 250 may use any structure known inthe art. The moving mechanism (not shown) includes a linear motor, etc.,and moves the mask 200 under control by the controller 500 by drivingthe mask stage 250 in the direction Y. The exposure apparatus 1synchronously scans the mask 200 and plate 400 through the controller500 for exposure.

The projection optical system 300 is a catoptric optical system thatprojects light from an object surface (such as the mask 200) onto animage surface (such as a surface of the object to be exposed, i.e., theplate 400) using reflections. The projection optical system 300 of theinstant embodiment exemplarily includes, as shown in FIG. 2, an opticalelement 310 that generalizes optical elements 310 a to 310 f, and aprojection control part 330. FIG. 2 is a schematic sectional view of theprojection optical system 300 of one embodiment according to the presentinvention.

The optical element 310 images the light using the reflections. Theoptical element 310 includes a mirror onto which a multilayer film thatreflects the EUV ray, and intensifies the light. A conceivablemultilayer that is applicable to the optical element 310 is, forexample, a Mo/Si multilayer film created by reciprocally laminating amolybdenum (Mo) layer and a silicon (Si) layer onto a reflection surfaceor a Mo/Be multilayer film created by reciprocally laminating amolybdenum (Mo) layer and a beryllium (Be) layer onto a reflectionsurface. For a wavelength range near a wavelength of 13.4 nm, theoptical element 310 composed of a Mo/Si multilayer film can obtain areflectance of 67.5%, and for a wavelength range near a wavelength of11.3 nm, the optical element 310 composed of a Mo/Be multilayer film canobtain a reflectance of 70.2%. However, the multilayer film applicableto the optical element 310 is not limited the above materials, and thepresent invention does not prevent an application of a multilayer filmthat has an operation and effect similar to those of the above.

Among the optical elements 310, third and fourth optical elements 310 cand 310 d in order of reflection of light from a side of the mask 200,have areas 312 c and 312 d for reflecting the light and are arrangedrotatable around the optical axis, as shown in FIGS. 3A and 4A. Theoptical element 310 d has a hole 318 d at its center through which thelight passes. Entire surfaces of areas 312 c and 312 d on the opticalelement 310 c and 310 d do not receive the exposure light (which meansherein light on an optical path from the mask to the image surface anddoes not mean light that necessarily contributes to exposure) duringexposure, and include local areas that receive the exposure light (i.e.,illuminated area) 314 c and 314 d, and areas that do not receive theexposure light (i.e., unilluminated area) 316 c and 316 d. When theilluminated areas 314 c and 314 d receive the exposure light, atemperature difference or distribution occurs between the illuminatedareas 314 c and 314 d and the unilluminated area 316 c and 316 d.However, when the optical elements 310 c and 310 d are rotated aroundthe optical axis, the entire surfaces of the areas 312 c and 312 d mayreceive the exposure light all around. This mitigates locality of thetemperature distribution, restrains a deterioration of the opticalperformance that results from a change of the surface shape of theoptical element 310, and obtains desired resolution.

As shown in FIGS. 3B and 4B, the cooling unit 320 may be provided andcool the areas 312 c and 312 d on the optical elements 310 c and 310 d,i.e., unilluminated areas 316 c and 316 d. The cooling unit 320 may coolthe areas 312 c and 312 d directly using the cooling liquid. Therefore,a temperature distribution that occurs in the optical elements 310 c and310 d may be restrained more effectively than those shown in FIG. 3A and4A. This may effectively restrain the deterioration of the opticalperformance that results from a change in surface shapes of the opticalelements 310 c and 310 d, and obtain the desired resolution. Here, FIGS.3 and 4 are plane views of exemplary optical elements 310 c and 310 d.FIGS. 3A and 4A show a case where the cooling unit 320 is not provided,while FIGS. 3B and 4B show a case where the cooling unit 320 isprovided.

Some of the optical elements 310 (e.g., first, fifth and sixth opticalelements 310 a, 310 e and 310 f in order from the side of the mask 200in this embodiment) cannot rotate around the optical axis due to theiraxially asymmetrical shape. Therefore, the optical elements 310 a, 310 fand 310 e cannot prevent surface-shape changes utilizing theaforementioned rotations around the optical axis. Accordingly, as shownin FIG. 5, the optical element 310 that may rotate around the opticalaxis (e.g., the optical elements 310 b, 310 c and 310 d in the instantembodiment) has been previously provided with areas 322 to 328 havingsurface shapes for correcting a deterioration of optical performancecaused by a change in surface shape of the optical elements 310 a, 310 fand 310 e. In other words, the areas 322 to 328 form surface shapes forcorrecting aberration that occurs when the surface shapes of the opticalelements 310 a, 310 f and 310 e that cannot rotate around the opticalaxis. The aberration includes at least one of a curvature of field,distortion and coma. The areas 322 to 328 are appropriately switched inaccordance with the aberration that occurs due to the deformation of theoptical elements 310 a, 310 f and 310 e. This may correct the aberrationthat occurs due to the deformation of the optical elements 310 a, 310 fand 310 e, which are not rotatable around the optical axis, by theoptical elements 310 c and 310 d that are rotatable around the opticalaxis, prevent the deterioration of the optical performance, and obtainthe desired resolution. FIG. 5 is a plane view of an optical elementthat is rotatable around an optical axis and has a surface shape forcorrecting a deterioration of another optical element due to asurface-shape change of the other optical element. Although FIG. 5 showsfour areas 322 to 328, the number and shapes are for illustrativepurposes only.

Optionally, the optical element 310 that receives the exposure light onits entire surface (e.g., the optical element 310 formed at the pupilposition in the instant embodiment) may be previously provided with asurface shape for generating astigmatism, and rotated to remove theastigmatism generated during exposure. Moreover, the optical element 310may be rotated to adjust the astigmatism at the time of assembling theprojection optical system 300.

A projection controller 330 includes a detector part 332, adetermination part 334, and a drive part 336. The detector part 332 isconnected to the optical element 310 in the projection optical system300 and determination part 334, and detects the exposure dose of theoptical element 310. The detected exposure dose of the optical element310 is transmitted to the determination part 334. The determination part334 is connected to the detector part 332 and the drive part 336, anddetermines based on data from the detector part 332 whether the opticalperformance the projection optical system 300 deteriorates. Thedetermination part 334 selects the optical element 310 and areas 322 to328 for correcting the deterioration of the optical performance of theprojection optical system 300, and transmits the information to thedrive part 336. The drive part 336 is connected to the determinationpart 334 and the optical element 310 that is rotatable around theoptical axis, and rotates the optical element 310 so as to realize theexposure using the optimal areas 322 to 328 on the optical element 310,which has been selected by the determination part 334.

A description will be given of the exemplary inventive exposure methodusing the projection optical system 300 with reference to FIG. 6. FIG. 6is a flowchart for explaining the inventive exposure method 1000. Whenthe exposure apparatus 1 starts exposure, the detector part 332 in theprojection controller 330 starts detecting the light amount of theexposure light (step 1002). Next, the exposure amount detected by thedetector part 332 is transmitted to the determination part 334, and thedetermination part 334 determines whether projection optical system 300causes deterioration of optical performance (step 1004). When thedetermination part 334 determines that no deterioration of the opticalperformance would occur, the exposure continues as it is (step 1006).When the determination part 334 determines that a deterioration of theoptical performance would occur, the optical element 310 that forms asurface shape for correcting deterioration of optical performance isselected (step 1008). The determination part 334 also selects optimalareas 312 to 318 for correcting a deterioration of optical performancefrom the surface shape of selected optical element 310. Then, the drivepart 336 rotates the selected optical element 310 and expose through theselected optimal areas 312 to 318 for correcting the deterioration ofthe optical performance (step 1012). Of course, the deterioration of theoptical performance in the projection optical system 300 may bedetermined based on the aberration in the projection optical system 300and an exposure result.

Turning back to FIG. 1, the plate 400 is an exemplary object to beexposed, such as a wafer and a LCD, and photoresist is applied to theplate 400. A photoresist application step includes a pretreatment, anadhesion accelerator application treatment, a photo-resist applicationtreatment, and a pre-bake treatment. The pretreatment includes cleaning,drying, etc. The adhesion accelerator application treatment is a surfacereforming process so as to enhance the adhesion between the photoresistand a base (i.e., a process to increase the hydrophobicity by applying asurface active agent), through a coat or vaporous process using anorganic film such as HMDS (Hexamethyl-disilazane). The pre-baketreatment is a baking (or burning) step, softer than that afterdevelopment, which removes the solvent.

The plate 400 is supported by the plate stage 450. For example, theplate stage 450 moves the plate 400 in XYZ directions. The mask 200 andplate 400 are scanned synchronously under control by the controller 500.The positions of the mask stage 250 and plate stage 450 are monitored,for example, by a laser interferometer and the like, so that both aredriven at a constant speed ratio.

The controller 500 includes a CPU and memory, which are not shown, andcontrols operations of the exposure apparatus 1. The control part 500 isconnected electrically to an illumination apparatus 100, (the movingmechanism (not shown) in) the mask stage 250, and (the moving mechanism(not shown) in) the plate stage 450. The CPU covers any processor,irrespective of its name, such as an MPU, and controls operations ofeach module. The memory includes a ROM and RAM, and stores firmware foroperations of the exposure apparatus 1. The controller 500 may serve asand be integrated with the projection controller 330.

In exposure, the EUV light emitted from the illumination apparatus 100illuminates the mask 200. The EUV light that reflects the circuitpattern on the mask 200 is imaged on the plate 400 by the projectionoptical system 300. The instant embodiment exposes the entire surface ofthe mask 200 by scanning the mask 200 and plate 400 at a speed of thereduction ratio.

The projection optical system 300 of this embodiment includes, in orderfrom the mask 200 to the plate 400, a convex aspheric catoptric element,a concave aspheric catoptric element, a convex aspheric catoptricelement, a concave aspheric catoptric element, and a convex asphericcatoptric element, but the present invention does not limit theprojection optical system 300 to this structure.

For example, the number of catoptric elements may be other than six, andthe power arrangement of the catoptric elements is not limited to aconvex, concave, convex, concave, convex and concave. Various projectionoptical systems are known in the art, and thus those skilled in the artwould arrange at least one catoptric element among plural catoptricelements around the optical axis rotatably in the known projectionoptical system based on the disclosure of the instant specification.

Unlike the above embodiments, the present invention does not limit theprojection optical system to the catoptric optical system, but may use aknown catadioptric projection optical system so that at least one ofplural refractive elements (i.e., lenses) and reflective elements (i.e.,mirrors) in the optical system may rotate around the optical axis toreduce the aberration.

The above embodiment has an object to correct aberrations generated inthe projection optical system due to thermal deformations ofnon-rotatable optical elements 310 a, 310 e, 310 f, etc., aberrationoriginally included in this projection optical system, and aberrationgenerated in the element 310 b that reflects light (i.e., exposurelight) on its entire surface although it is not rotatable. In order tocorrect these aberrations, one or more predetermined reflection areasare selected in one or more of reflection surfaces in the rotatableoptical elements 310 b, 310 c, and 310 d. In selecting the predeterminedreflection area(s) in the plural optical elements, a combination ofplural reflection areas may be selected in plural optical elements.

Referring to FIGS. 7 and 8, a description will now be given of anembodiment of a device fabricating method using the above exposureapparatus 1. FIG. 7 is a flowchart for explaining a fabrication ofdevices (i.e., semiconductor chips such as IC and LSI, LCDs, CCDs,etc.). Here, a description will be given of a fabrication of asemiconductor chip as an example. Step 1 (circuit design) designs asemiconductor device circuit. Step 2 (mask fabrication) forms a maskhaving a designed circuit pattern. Step 3 (wafer preparation)manufactures a wafer using materials such as silicon. Step 4 (waferprocess), which is referred to as a pretreatment, forms actual circuitryon the wafer through photolithography using the mask and wafer. Step 5(assembly), which is also referred to as a posttreatment, forms into asemiconductor chip the wafer formed in Step 4 and includes an assemblystep (e.g., dicing, bonding), a packaging step (chip sealing), and thelike. Step 6 (inspection) performs various tests for the semiconductordevice made in Step 5, such as a validity test and a durability test.Through these steps, a semiconductor device is finished and shipped(Step 7).

FIG. 8 is a detailed flowchart of the wafer process in Step 4 shown inFIG. 7. Step 11 (oxidation) oxidizes the wafer's surface. Step 12 (CVD)forms an insulating film on the wafer's surface. Step 13 (electrodeformation) forms electrodes on the wafer by vapor disposition and thelike. Step 14 (ion implantation) implants ion into the wafer. Step 15(resist process) applies a photosensitive material onto the wafer. Step16 (exposure) uses the exposure apparatus 200 to expose a circuitpattern on the mask onto the wafer. Step 17 (development) develops theexposed wafer. Step 18 (etching) etches parts other than a developedresist image. Step 19 (resist stripping) removes disused resist afteretching. These steps are repeated, and multilayer circuit patterns areformed on the wafer. The device fabrication method of this embodimentmay manufacture higher quality devices than the conventional one, byrestraining a surface-shape change of the optical element in theprojection optical system and correcting a deterioration of the opticalperformance. Thus, the device fabrication method using the exposureapparatus 1, and the devices as finished goods also constitute oneaspect of the present invention.

Further, the present invention is not limited to these preferredembodiments, and various variations and modifications may be madewithout departing from the scope of the present invention. For example,the number of optical elements in the projection optical system of thisembodiment is not limited to six. The present invention is applicable toan exposure that uses a UV light source having a wavelength less than200 nm other than EUV light, such as ArF excimer laser and F₂ laser, anda step-and-repeat exposure apparatus (i.e., stepper).

Thus, the inventive projection optical system, and an exposure apparatusand method may obtain desired resolution, and provide superior exposureperformance

1. A projection optical system comprising: a first mirror that isrotatable around an optical axis of the projection optical system; and asecond mirror that is not rotatable around the optical axis, wherein thefirst mirror includes a first reflective area and a second reflectivearea different from the first reflective area, wherein the first mirrorreflects light in the first reflective area or the second reflectivearea when rotating around the optical axis, wherein the first mirrorreflects the light in the first reflective area when a surface shape ofthe second mirror does not change, and the first mirror reflects thelight in the second reflective area when the surface shape of the secondmirror changes, and wherein a surface shape of the second reflectivearea of the first mirror is different from that of the first reflectivearea, and is configured to reduce an aberration that occurs as thesurface shape of the second mirror changes.
 2. A projection opticalsystem according to claim 1, wherein the aberration includesastigmatism.
 3. A projection optical system according to claim 1,wherein the aberration includes at least one of a curvature of field,distortion and coma.
 4. An exposure apparatus comprising: anillumination optical system configured to illuminate a mask by utilizinglight from a light source; and a projection optical system according toclaim 1 configured to project a pattern of the mask onto a wafer.
 5. Adevice manufacturing method comprising the steps of: exposing a wafer byutilizing an exposure apparatus according to claim 4; and developing thewafer that has been exposed.